The batch processing of micromachining has led to the emergence of capacitive micromachined transducers. These transducers offer a larger set of parameters for optimization of performance as well as ease of fabrication and electronic integration. The fabrication and operation of micromachined transducers have been described in many publications and patents. For example, U.S. Pat. Nos. 5,619,476, 5,870,351, 5,894,452 and 6,493,288 describe the fabrication of capacitive-type ultrasonic transducers. U.S. Pat. Nos. 5,146,435; 5,452,268, and 6,870,937 also describe micromachined capacitive transducers that are mainly used in the audio range for sound pickups. In most structures, the movable diaphragm of a micromachined transducer is either supported by a substrate or insulative supports such as silicon nitride, silicon oxide and polyamide. The supports engage the edge of membrane, and a voltage is applied between the substrate and a conductive film on the surface of the membrane causes the membrane to vibrate in response to the passing sound waves. In one particular case as described in the U.S. Pat. No. 6,535,460, the diaphragm is suspended to allow it rest freely on the support rings.
Many micromachined condenser microphones use a similar membrane structure to that of large measurement microphones and studio recording microphones. One common structure, shown in FIG. 1, consists of a conductive membrane 1 suspended over a conductive back-plate 5 that is perforated with acoustic holes 3. Sound detection is possible when the impinging pressure wave vibrates the membrane 1, thus changing the capacitance of the transducer 2. Under normal operation, the change in capacitance of the condenser microphone 2 is detected by measuring the output current 8 under constant-voltage bias. A pressure equalization vent 4 in the back-chamber 7 prevents fluctuations in atmospheric pressure from collapsing the membrane 1 against the back-plate 5. A precision condenser microphone for measurement or calibration applications is capable of a uniform frequency response due to its relatively large air gap, on the order of 20 μm, behind the membrane. Silicon micromachined microphones, with membrane dimensions of 1–2 mm, require air gaps 6 on the order of a few microns to maintain adequate sensitivity due to the reduced motion that results from a smaller membrane. However, the reduced dimensions of the air gap magnify the effects of squeeze-film damping, introducing frequency-dependent stiffness and loss. This creates undesirable variations in the mechanical response with acoustic frequency. Furthermore, achieving a large dynamic range and a high sensitivity can be conflicting goals, since large sound pressures may cause the membrane to collapse under its voltage bias. This traditional approach suffers from low sensitivity, especially at low frequencies.
In order to achieve wide bandwidth and high sensitivity, the development of high-performance diaphragm is of vital importance in the successful realization of condenser microphones. For most very thin diaphragms, however, large residual stress can lead to undesirable effects such as low and irreproducible performances, if the processes cannot accurately be controlled. One technique for acquiring low-stress diaphragms is to use a sandwich structure, in which layers with compressive and tensile stress are combined. Another technique is to use the support structure such as outlined in the U.S. Pat. No. 6,847,090. U.S. Pat. No. 6,535,460 also describes a structure that the membrane is freely suspended to allow it release the mechanical stress. Unfortunately, in this case, the freely suspended membrane will have unstable sensitivity and unwanted lateral movement, resulting in the signal spew and posing the reliability issues.